Asymmetric phenomenon of flow and heat transfer in charging process of thermal energy storage based on an entire domain model

TL;DR

This paper investigates the asymmetric flow and heat transfer phenomena during the charging process of phase change energy storage, emphasizing the importance of using an entire domain model for accurate simulation and understanding.

Contribution

It introduces the use of an entire domain model and nonlinear dynamics to explain asymmetry in phase change energy storage, revealing the role of natural convection and proposing methods to enhance charging speed.

Findings

Natural convection causes multiple asymmetric solutions.

Asymmetry exists even in symmetric geometries.

Adding thermal disturbance can improve charging efficiency.

Abstract

Phase change energy storage is getting increasing attention as a representative technology to achieve carbon neutrality. The phase change process exists typical phenomenon of asymmetry that affects the energy storage performance. However, the mechanism of asymmetry is currently lack of elaboration because the half domain model is always used to simplify the numerical simulation and avoid the appearance of asymmetry. In this study, the entire domain model and boundary conditions were adopted and numerically simulated for the melting process of paraffin wax, i.e., the charging process of energy storage. The nonlinear dynamics method was applied to explain the asymmetric flow and heat transfer phenomenon. The three-stage characteristic based on charging speed was proposed for charging process and was explained by thermal conduction or natural convection. The asymmetric phenomenon was elaborated on the cause of formation, change mechanism, and effect evaluation. Results show that natural convection accounts for the multiple solutions (including the asymmetric ones) of charging process. It is also pointed out that asymmetric solutions can exist under symmetric geometry structure. To accurately solve the charging process, it is necessary to use the entire domain model. Both the charging speed and energy storage capacity have a positive correlation with asymmetry. Thus, a potential idea for energy storage application was proposed to increase the charging speed; that is, adding thermal disturbance during the melting process to destroy flow stability and to enhance convective heat transfer. The research methods and findings will support the development of energy storage and numerical investigation in other related areas.